1. Field of the Invention
This invention pertains to the removal of sulfur values in the treatment of an alkali metal sulfide liquor. It particularly relates to a method wherein the alkali metal sulfide is converted to the corresponding alkali metal carbonate and the sulfur is recoverable as a salable product. In a particularly preferred aspect, the invention relates to the treatment of a liquor containing a solute which comprises a mixture of at least about 60 mole % alkali metal sulfide and the balance is principally alkali metal carbonate.
2. Prior Art
There are several sources of mixed alkali metal salts such as, for example, mixtures of alkali metal sulfides and carbonates, which advantageously are treated to recover the sulfur content and produce a substantially pure alkali metal carbonate. One such source is the pulp and paper industry, wherein a cellulose is manufactured by digestion of wood with alkali metal sulfides or sulfite. In such a process, a bleed stream digestion liquor is drawn off, concentrated, and burned producing a molten salt mixture which generally consists substantially of sodium carbonate and sodium sulfide. Various methods have been suggested for treating such molten salts.
More recently, it has been suggested that carbonaceous materials such as coal, coal tar, oil shale, petroleum coke, and petroleum residuums, be decomposed in a molten alkali metal carbonate bath to recover the resultant valuable gaseous products. The feed materials generally contain sulfur compounds, which react with and are retained in the molten bath. In addition, such materials, particularly in the case of coal, also contain significant amounts of inorganic ash constituents, which also are retained in the molten bath. Therefore, it is necessary to periodically (or continuously) remove a portion of the molten alkali metal carbonate bath and treat it to remove the retained ash constituents and sulfur, and permit the return of alkali metal carbonate to the bath for further use.
Another source of mixed alkali metal sulfide-carbonate salts is from a gas scrubbing process, wherein a waste gas containing sulfur oxides is reacted with alkali metal carbonates and/or bicarbonate absorbent (either dry or in an aqueous solution) to produce an alkali metal sulfite and sulfate product which generally also contains some unreacted absorbent. The alkali metal sulfite and sulfate product can be reacted with a reducing agent such as carbon or coal to form alkali metal sulfides which then can be introduced into an aqueous medium to form an aqueous solution of alkali metal sulfides, which may also contain alkali metal carbonate, bicarbonate, and any ash constituents removed from the waste gas stream or contained in the reducing agent utilized.
In U.S. Pat. No. 2,094,070, Hultman et al., there is disclosed a process for recovering H.sub.2 S from gases. Broadly, the process comprises contacting an H.sub.2 S-containing gas stream with a solution of alkali metal carbonate, which absorbs the hydrogen sulfide. The solution then is treated with a sufficient amount of carbon dioxide to convert the carbonate into bicarbonate without removal of the hydrogen sulfide. The treated solution is boiled under a vacuum to expel the hydrogen sulfide and steam. The boiling is then continued at a higher temperature and pressure to convert the bicarbonate back to the carbonate for recycle to absorb more H.sub.2 S.
U.S. Pat. No. 2,496,550, Larsson et al., discloses a process for the recovery of alkali metal salts from the waste liquors from the production of cellulose by the digestion of the wood with an alkali metal salt such as alkali metal sulfites and bisulfites. The process comprises evaporating and burning the waste liquor to form a molten mass containing alkali and sulfur compounds followed by dissolving the molten mass in a solvent. Thereafter, carbon dioxide is introduced into the solution to form alkali metal bicarbonate in an amount sufficient to approach the limit of solubility of the bicarbonate in the solution. The solution is then heated to eliminate the hydrogen sulfide formed therein. After removal of the hydrogen sulfide, the solution is treated with additional carbon dioxide to crystallize alkali bicarbonate, which is recovered from the solution. Larsson et al further suggests that the solution, after removal of the alkali metal bicarbonate, be used as the solvent for dissolving the molten mass formed from burning the waste liquor.
U.S. Pat. No. 2,675,297, Gray et al., relates to the treatment of aqueous solutions of sodium sulfide for conversion of the sulfides to salts of carbonic acid and the liberation of hydrogen sulfide. Patentees suggest subjecting a solution containing sodium sulfide to a plurality of carbonaceous treatments with gaseous carbon dioxide at an elevated temperature and pressure, each carbonation treatment being followed by steam stripping under a vacuum to remove volatile hydrogen sulfide in a concentrated form, thereby producing a solution having an enhanced content of sodium salts of carbonic acid substantially free from sulfide.
In U.S. Pat. No. 3,567,377, Lefrancois et al., there is disclosed a process for the recovery of sulfur values from sulfur-bearing materials. In accordance with the process disclosed therein, a sulfur-containing carbonaceous material is contacted in the presence of a reducing gas with a molten medium comprising an alkali metal carbonate to convert the sulfur to an alkali metal sulfide. The molten medium containing the absorbed alkali metal sulfide is mixed with an aqueous solution of the acid salt of the alkali metal carbonate. The resulting solution is filtered to remove any solids contained therein and then reacted with carbon dioxide to form hydrogen sulfide as a gaseous product of the reaction.
U.S. Pat. No. 3,508,863, Kiminki et al., discloses a process for the preparation of sodium carbonate monohydrate from a soda smelt solution. The process is directed toward the treatment of a smelt solution arrived at by burning spent pulping liquor. The smelt solution is precarbonated with gases containing carbon dioxide to form dissolved sodium bicarbonate followed by evaporation of the precarbonated solution together with added sodium bicarbonate to remove all the sulfides as hydrogen sulfide together with water vapor and to form crystalline sodium carbonate monohydrate for recovery.
Other patents relating to the treatment of alkali metal sulfides are U.S. Pat. No. 1,945,163 (Rosenstein et al.); U.S. Pat. No. 2,730,445 (Sivola); U.S. Pat. No. 3,438,728 (Grantham); U.S. Pat. No. 3,574,543 (Heredy); and U.S. Pat. No. 3,867,514 (Moore).
While each of the foregoing processes offer certain advantages, none has proven entirely satisfactory. Some are disadvantageous in that a multitude of steps are required. In others, no provision is made for obtaining the necessary carbon dioxide from the process itself, thus raising the operating cost, while others create difficulties owing to corrosion and very high amounts of gases to be handled. Several of the processes require use of excessive amounts of steam, and others require use of heat exchangers which are subject to plugging and scaling due to the deposition of solid materials from solution. In others, noxious sulfur-containing gases are evolved to the atmosphere. A principal problem with most of the prior art processes for treating sulfide liquors to remove the sulfur values therefrom is that they either (a) produce a dilute (less than 30 vol.%) H.sub.2 S gas stream, or (b) when they produce an H.sub.2 S-rich gas stream, i.e., one containing in excess of 30% H.sub.2 S on a dry basis, the processes require the use of substantially pure CO.sub.2. When the H.sub.2 S gas stream is dilute, it necessarily requires more expensive processing to recover the sulfur values. For example, in the operation of a Claus plant, when the feed stream of H.sub.2 S is less than about 30%, an auxiliary source of fuel is required, thus obviously increasing the cost of the process. Also, a dilute Claus feed gas requires a greater number of stages to obtain a high percent of sulfur recovery. When the H.sub.2 S gas stream contains more than 30% by volume on a dry basis of H.sub.2 S, then an auxiliary source of fuel is not necessary, and the size of the plant can be reduced. Obviously, this is desirable both from the standpoint of economics and in the interest of conserving fuel. There is, therefore, a need for a method of treating sulfide liquors to produce an H.sub.2 S offgas containing greater than 30% by volume H.sub.2 S, and which method does not require the use of pure CO.sub.2. Obviously, therefore, there is still a need for an improved economical process for the treatment of mixtures of alkali metal sulfides and carbonates for the removal of sulfur values therefrom.